GEOPHYSICAL RESEARCH LETTERS, VOL. 34, L05105, doi:10.1029/2006GL028932, 2007 Click Here for Full Article Anomalous cosmic rays in the distant heliosphere and the reversal of the Sun’s magnetic polarity in Cycle 23 F. B. McDonald,1 E. C. Stone,3 A. C. Cummings,3 W. R. Webber,2 B. C. Heikkila,4 and N. Lal4 Received 28 November 2006; revised 5 January 2007; accepted 17 January 2007; published 7 March 2007. [1] Beginning in early June 2001 and extending over a where, by the still generally accepted paradigm, they are 3 month period the Cosmic Ray Subsystem (CRS) accelerated at the TS [Fisk et al., 1974; Pesses et al., 1981]. experiment on Voyager 1 (80.3 AU, 34°N) observed a Their relative charge composition of H, He, N, O, Ne and A rapid transition in the energy of the peak intensity of along with a small C abundance - when corrected for anomalous cosmic ray (ACR) He from 11 MeV/n to ionization rates, filtration effects and acceleration efficiency 25 MeV/n. One month later there began a similar spectral - is very similar to the abundance of neutral atoms in the shift at Voyager 2 (64.7 AU, 24°S). When these ACR local interstellar medium and their energy spectra are transition times are extrapolated back from the estimated consistent with that expected from shock acceleration at location of the heliospheric termination shock (TS) to the the TS [Cummings et al., 2002]. The heliospheric TS, which Sun (1 year) there is reasonable agreement with the range was crossed by Voyager 1 (V1) on 16 Dec. 2004 at 94 AU, of times obtained for the polarity reversal of the represents an abrupt transition in the solar wind flow when interplanetary magnetic field (IPB) in the northern the ram pressure of the expanding solar wind reaches hemisphere near the Sun and the later reversal in the approximately that of the local interstellar medium. south. We interpret these changes in the ACR spectra as [4] Near solar minimum the IPB field reflects the major giving a global view of the interplanetary field reversal at dipole component of the Sun’s magnetic field and consists the TS and confirmation of the important role of the TS in of hemispheres separated by a near-equatorial, thin helio- controlling the ACRs. Citation: McDonald, F. B., E. C. Stone, spheric current sheet (HCS) with the field being oppositely A. C. Cummings, W. R. Webber, B. C. Heikkila, and N. Lal directed on either side of the sheet. When the open field (2007), Anomalous cosmic rays in the distant heliosphere and the lines in the northern hemisphere are outward directed from reversal of the Sun’s magnetic polarity in Cycle 23, Geophys. Res. the solar surface (positive polarity, A > 0), ACRs drift and Lett., 34, L05105, doi:10.1029/2006GL028932. are accelerated along the TS away from the equatorial region and toward the Northern and Southern polar regions. 1. Introduction They then drift from these polar regions down through the heliosphere along with positively charged GCR ions. With [2] Galactic and anomalous cosmic rays (GCRs and increasing solar activity the inclination of the HCS ACRs) probe the large-scale structure and dynamics of increases, reaching 90° near solar maximum. In A < 0 our heliosphere. In this study we use the Voyager observa- epochs this drift pattern is reversed. In this note we study tions of these energetic particles in the distant heliosphere to the effects of the cycle 23 reversal of the solar magnetic study the times of the reversal of the Sun’s magnetic polarity on ACR helium. polarity as reflected in the changes in the IPB field at the heliospheric TS over the recent solar maximum period of cycle 23 (2000–2001). 2. Observations [3] Below some 50 MeV/n, ACRs are the dominant [5] The V1 and V2 time histories of GCR He and ACR energetic particle population in the outer heliosphere He+ (Figure 1) from 1996.0–2006.25 show the small [Cummings et al., 2002]. These predominantly singly- variation of GCR He from solar minimum to solar maxi- charged particles [Klecker et al., 1995, Mewaldt et al., mum in the distant heliosphere and the larger temporal 1996] have their origin as interstellar neutral atoms which variation of the ACRs. There are no obvious features in have been ionized either by solar ultraviolet photons or these time histories that would be ascribed to the polar through charge exchange with solar wind ions. These magnetic field reversals from A > 0 to A < 0 over the solar singly-charged pick-up ions are then convected out to the maximum period of cycle 23. distant heliosphere by the outward flowing solar wind [6] However a V1 regression plot of the intensity of 15.5 MeV/n He+ versus that of 43 MeV/n He+ (Figure 2a) reveals a straight line fit from 1997.0–2001.35 followed by 1Institute for Physical Science and Technology, University of Maryland, a 95 ± 12 day transition to a new regression line (from College Park, Maryland, USA. 2001.64 ± .03 to 2004.21 ± .04). This line is well defined 2Department of Physics and Astronomy, New Mexico State University, until 2004.21 when there is increased dispersion perhaps Las Cruces, New Mexico, USA. due to the passage of strong interplanetary transients. There 3Space Radiation Laboratory, California Institute of Technology, Pasadena, California, USA. is also a significant change in the relative behavior of the 4NASA Goddard Space Flight Center, Greenbelt, Maryland, USA. two ACR components some 6 months after V1 entered the heliosheath. The V2 ACR He+ follow a similar pattern Copyright 2007 by the American Geophysical Union. except the transition occurs a month later over a 128 day 0094-8276/07/2006GL028932$05.00 L05105 1of5 L05105 MCDONALD ET AL.: ACRS AND THE MAGNETIC POLARITY OF THE TS L05105 spectral peak from 2001.85 to 2006.21 despite the x25 increase in 15.5 MeV/n He+. 3. Analysis [8] We interpret these ACR spectral transitions as mark- ing the time of the reversal of the magnetic field at the TS near the nose region of the heliosphere. The next step is to relate the transition times at the TS back to the Sun. Due to the effects of pick-up ions there is deceleration with increas- ing heliocentric distance which Wang and Richardson [2003] have shown to be on the order of 1.1 km/s/AU between 1 and 60 AU and in their model this deceleration remains constant out to the TS. The measured V2 daily averages of the solar wind speed are used to find the time when the solar wind associated with a given transition at the TS passed V2 (Table 1, column C). This V2 time is then extrapolated back to the Sun (Table 1, column F). [9] There is good agreement between the V2 observed values of the solar wind speed, V, and the inferred speed at V1, using the transit time of MeV solar energetic particle events [Richardson et al., 2005, McDonald et al., 2005]. The location of the TS over this period has been estimated by Richardson et al. [2006] and Webber [2005] using the plasma pressure measured by V2 and in the case of Richardson et al. [2006], normalizing to the crossing of the TS at 94 AU on 16 Dec. 2004. The values of V used in Figure 1. Voyager-1 (solid circles) and Voyager-2 (open triangles) 26 day average time history of GCR and ACR He. The 30–56 and 10–21 MeV/n He+ have been corrected for GCR He but not for the lower energy termination shock particles. The top brackets in each panel mark the time of termination shock particle events at V1 [McDonald et al., 2005]. The shaded rectangles mark the observed transition from positive to negative polarity at V1 and V2 while the vertical dashed line shows the time V1 crossed the termination shock. All of the particle data in this paper are from the Voyager CRS experiment [E.C. Stone, P.I.] period (Figure 2b). Similar transition times were obtained for 8 MeV/n He+ versus 43 MeV/n He+. A regression plot of the intensity of higher rigidity GCRs versus that of 30– 56 MeV/n He+ (Figure 2c) gives a straight line fit from 1997.0–2006.21 and yields the same transition times as the lower energy He+ components. [7] From the V1 and V2 energy spectra just before and after the transition (Figures 3a and 3b) it can be seen that the main effect of the polarity reversal is a suppression of the particles below the spectral peak and at V1 a modest increase in the particle intensity above the peak region. This difference between the spectral changes at V1 and V2 leads to different trajectories in the hysteresis plots over the transition period (Figures 2a and 2b). At both spacecraft the spectral peak shifts from 6.5 MeV/n at solar minimum to 11 MeV/n at solar maximum to 25 MeV/n immediately after Figure 2. Regression plot of 10–21 MeV/n He+ (V1, V2) the transition (Figures 3c and 3d). It remains at this higher and 300–450 MeV/n GCR He (V1) versus 30–56 MeV/n energy at V1 but over time it gradually merges with the lower He+ for the periods shown in Figure 1. The top solid lines energy termination shock particle population [Cummings et are least square fits to the data from solar minimum to solar al., 2006; Hill et al., 2006].